Several arguments in Aristotle's Nicomachean Ethics – in particular Chapter 9 – suggest Aristotle thinks virtuous individuals need friends because, as a second self, friends provide a route to self-knowledge. That sounds correct, but seems too limited. Consider the following: During a long-term relationship years ago, I tried in vain to be friendly with my lover's best friend, call her E. E and I did not get along, and were around each other often. Reflection on why suggested to me that E and I shared many qualities, but those qualities we shared were qualities I didn't like about myself, e.g. pride, aggressiveness, etc. Meeting with E reminded me of what I disliked about myself. In fact, I learned quite a bit about myself that I didn’t like from E, who I wouldn’t consider a friend.
Now, neither of us was virtuous, and it doesn’t seem Aristotle would count this relationship as a friendship, e.g. neither of us felt goodwill towards the other, etc. Nevertheless, E provided an effective route to self-knowledge because she was, in a sense, a second self – reflecting vicious qualities of mine at the time. This leads me to think where Aristotle thinks virtuous individuals need friends because their second-self provides self-knowledge, less-than-virtuous individuals might need less-than-virtuous relationships – not necessarily friendships - because these second-selves also provide self-knowledge.